Prevent ASP.NET from redirecting to login.aspx

We have a website that runs fully on AngularJS with an ASP.NET Web API back-end with the following configuration:
- HTML5 routing is enabled on Angular and there is a rewrite rule in web.config to direct all traffic to index.html
- MVC is not installed (only razor pages)
- Authentication happens using Forms Authentication and related cookies

I have just added the Helicon IIS plugin to have .htaccess password protected for our development server (it's a mess to do it with IIS alone) but I have a basic problem.

After I input the basic auth credentials, I will get a redirect to

/login.aspx?ReturnUrl=

and although I'm not sure who is responsible for this (IIS or Helicon plugin), it will match one of my AngularJS routes and result in an error.